s = input()
mx = [0]
if len(s) == 4:
    a = [s[0], s[1], s[2], s[3], s[0:2], s[1:3], s[2:4], s[0:3], s[1:4], s]
    for j in range(len(a)):
        n = int(a[j])
        c = 0
        for i in range(1, n+1):
            if n % i == 0:
                c += 1
        if c == 2:
            mx.append(n)
print(max(mx))
